In starter/generator systems (for example, in aircraft), it is highly desirable to use a machine where the excitation can be removed, to safely shut off operation of the generator function and thus prevent excess heat in failures or smoke generation. Removing excitation from the machine provides a positive shut-off of electrical power dissipation without the need to shut down the prime mover engine that also is used for propulsion.
A DC brushed machine is often used for this application, due to its ability to act as a motor or generator with minimal control electronics and its ability to remove most of the field excitation by reducing the field winding current to zero. Alternatively, an induction machine may be used. An induction machine has the advantage of low cost and does not employ brushes. In addition, it can control the excitation to ensure safety and maximize utilization of the interfacing power electronics.
The ability to remove excitation is also a disadvantage in that it is also highly desirable to be able to initiate generation with no external power source other than shaft power, so that each generator is failure independent of another. In addition, when an induction machine is used for a motor or a generator, a speed reference signal is often needed to provide control.
Accordingly, it is desirable to implement a combined sensor and power source which provides both the needed speed sensing and control power to allow an induction machine generation system to initiate generation with no external power input.